Conventional coating materials for road markings are classified into JIS K5665 Class 1 (ordinary temperature), Class 2 (heating) and Class 3 (melting). The coating material for road markings is used for marking boundary lines or road signs (hereinafter simply referred to as "road markings") and mainly required to have good drying property, adhesive strength to the road surface, abrasion resistance, staining resistance and the like.
The coating materials of Class 1 and Class 2 are predominantly used for marking outer lines and lane boundaries. These coating materials use a ketone-type, ester-type, aliphatic or aromatic solvent having a high evaporation rate and since the coated film is formed by volatilizing the solvent, there are serious problems in view of safety and air pollution. In order to solve these problems, coating materials for road markings using an aqueous coating material have been proposed (see, JP-A-61-243866 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-3-157463 and JP-A-6-271789), however, water used as a solvent in these coating materials is low in the drying rate and accordingly, these coating materials for road markings are difficult to have quick drying property required.
The coating material of Class 3 is predominantly used for marking zebra zones and center lines. This is a melting-type coating material which is used after melting at from 180 to 220.degree. C., and therefore, this coating material has super-quick drying property and is free of the problem of air pollution. However, in the case of the melting-type coating material, a petroleum resin is generally used as a binder component and since the petroleum resin itself is poor in the abrasion resistance, this coating material is disadvantageously high in the rate of abrasion due to abrading by tires of running cars. In order to maintain the function as a road marking for a necessary time period, the coated film must be formed to have a fairly large thickness of 1 mm or more. For forming a film to have a large thickness, a huge amount of coating material is consumed, the transportation cost of the coating material increases, and the coating work and the like require great labors. Furthermore, a heat source for heat-melting the coating material must be carried into the field and working in a high-temperature environment is unavoidable. Thus, improvements have been demanded from the standpoint of safety and health of workers.
In recent years, photoradical polymerization and photocationic polymerization are drawing attention in view of the quick drying property, ordinary temperature curability, availability of a solvent-free system and energy savings, and these are practically used in various fields such as printed board fabrication, resist or photomask, wood painting, optical fiber coating, hard coating on plastics and can coating. However, since in almost all cases, the light used for the curing has a wavelength in an ultraviolet region of 400 nm or less, the material poorly transmits the light and as in the case of a coating material for road markings, when a film is formed to have a large thickness using a coating material containing a pigment having high covering power, such as titanium oxide, the film is very difficult to cure. To solve this problem, JP-A-8-209058 has attempted to use an acylphosphine oxide-type photopolymerization initiator which is sensitized by a visible ray at from 400 to 450 nm having a relatively high permeability through a material and generates a radical. However, since JP-A-8-209058 uses a low boiling point monomer such as methyl methacrylate, malodors and air pollution are very likely caused. Moreover, the methyl methacrylate is very low in the photocuring rate and the coating material for road markings formed is difficult to photocure in seconds.
The present inventors have previously proposed a photopolymerizable initiator capable of curing a material containing a pigment and having a large thickness (see, JP-A-6-75374 and JP-A-10-81838). According to these techniques, a radical polymerization initiator comprising a cationic dye having an absorption in the visible ray or near infrared ray region and a quaternary organic borate-type sensitizer is added to an ultraviolet ray radical polymerization initiator and thereby high curability can be achieved not only on the surface of the coated film but also in the inside of the photocurable material containing a pigment. However, it is not known to add a specific filler and use the initiator in a paint composition for road markings.